3 replies to this topic

[Double D]

Hi Folks,
I'm trying to determine the specific gravity of a different ratio's containing the following chems.
H20 @ 12.5ml
Ch3OH @ 100.5ml
H2SO4 @ 3.15ml

Is there a formula available that I can use to predict the SG of those Chems at different ratios?
Because naturally the combine w/v alone won't work.

[breizh]

Hi Folks,
I'm trying to determine the specific gravity of a different ratio's containing the following chems.
H20 @ 12.5ml
Ch3OH @ 100.5ml
H2SO4 @ 3.15ml

Is there a formula available that I can use to predict the SG of those Chems at different ratios?
Because naturally the combine w/v alone won't work.

1) No formula , only lab determination ( take account the temperature during experimentation)
2) specific gravity or density /

Hope it helps

Breizh

[MrShorty]

Is there a formula available that I can use to predict the SG of those Chems at different ratios?

How accurately do you need to predict them? How useful any predictive equation is depends a good deal on how accurately in predicts the quantity in question, and how accurately you need to know that quantity. Where this is posted in the Student forum, I'm guessing this is a homework problem, or part of a homework problem. They may not directly specify an accuracy, but there should be some indication from the context of the problem/chapter just what you are supposed to be learning.

Because naturally the combine w/v alone won't work.

Related to the previous question, I'm going to play "Devil's advocate" and ask: why can't you assume volumetric additivity? I'm guessing it has to do with the context of the homework problem.

Let me illustrate: I took density data from Perry's for methanol+water mixtures at 20 C. Then I calculated a simple weight fraction average of the specific volumes (specific volume=1/density) and compared the two. At worst, my simple prediction was in error by about 3.5% in the middle of the composition range.

I then repeated the exercise with sulfuric acid+water mixtures. The max error was greater for this binary (nearly 10%), but those larger errors were towards the concentrated sulfuric acid end. For dilute H2SO4 mixtures (<20%), the error was well within 5%.

So, the answer to your question in part comes down to how accurately do you need to predict the density. If you need very accurate results, then, as breizh pointed out, you need some real experimental data (and even then, you'll only be as accurate as the data you are basing your calculations on). If, within the context of the larger problem at hand, you can tolerate some error in your prediction, then you can find some different ways to estimate the density. Simple volumetric additivity is the easiest calculation. The Properties of Gases and Liquids has a chapter devoted to volumetric properties of mixtures where they discuss a couple of different approaches to estimate liquid densities.

[Double D]

Is there a formula available that I can use to predict the SG of those Chems at different ratios?

How accurately do you need to predict them? How useful any predictive equation is depends a good deal on how accurately in predicts the quantity in question, and how accurately you need to know that quantity. Where this is posted in the Student forum, I'm guessing this is a homework problem, or part of a homework problem. They may not directly specify an accuracy, but there should be some indication from the context of the problem/chapter just what you are supposed to be learning.

Because naturally the combine w/v alone won't work.

Related to the previous question, I'm going to play "Devil's advocate" and ask: why can't you assume volumetric additivity? I'm guessing it has to do with the context of the homework problem.

Let me illustrate: I took density data from Perry's for methanol+water mixtures at 20 C. Then I calculated a simple weight fraction average of the specific volumes (specific volume=1/density) and compared the two. At worst, my simple prediction was in error by about 3.5% in the middle of the composition range.

I then repeated the exercise with sulfuric acid+water mixtures. The max error was greater for this binary (nearly 10%), but those larger errors were towards the concentrated sulfuric acid end. For dilute H2SO4 mixtures (<20%), the error was well within 5%.

So, the answer to your question in part comes down to how accurately do you need to predict the density. If you need very accurate results, then, as breizh pointed out, you need some real experimental data (and even then, you'll only be as accurate as the data you are basing your calculations on). If, within the context of the larger problem at hand, you can tolerate some error in your prediction, then you can find some different ways to estimate the density. Simple volumetric additivity is the easiest calculation. The Properties of Gases and Liquids has a chapter devoted to volumetric properties of mixtures where they discuss a couple of different approaches to estimate liquid densities.

Mr. Shorty's and breizh, Thank you for the reply's.
As accurately as possible would be nice but <2% would be preferred.
I'm not a student or Chem engineer, I just posted this to the student forum because I guessed it's a question a lesser experienced person may ask.
Volumetric additivity won't work because adding the Chems as listed gives you a SG of .849 but actually weighing that sample gave me a SG of .944 way to large an error.
I guess I should have listed,
CH3OH at 99% purity
H2SO4 at 96% purity
H2O at ?% purity

What I'm trying to do is perform esterification reactions of mixtures containing Oleic acid with methanol using H2SO4 catalyst.
I would like to be able to keep the SG of the excess reactants, reagents and the H2O produced to >.94

It sounds like I may need to do some more tests.
Thank you so much for your help.